This invention relates generally to sheet delivery systems and particularly to apparatus for holding a large stack of sheets on a horizontal plate for delivery one at a time to an apparatus such as a photocopier.
Most photocopiers in use today have systems for storing a stack of transfer sheets from which the sheets are fed, one at a time, to the photocopier for image transfer. Typically the sheet stack is held on a movable platform, so that as sheets are removed from the stack the feed position of the top sheet is held nearly constant. Feed rollers mounted in the system engage the top sheet at this fixed feed position. An example of such a system is shown in U.S. Pat. No. 4,032,137. The patent shows springs maintaining a bias on a plate on which the transfer sheets are stacked. The springs are selected so that as sheets are fed from the stack, the feed position of the top sheet remains the same.
The major problem in any sheet feeding apparatus is feeding sheets from the stack one at a time. This problem usually manifests itself in two sheets (or more) being stripped from a stack rather than one, with the multiple sheets being either in exact registration or with one slightly trailing the other (the shingle effect). The problem may be caused by slight variations in paper size, thickness, and texture, or by variations in ambient conditions such as humidity. Another problem in feeding sheets is maintaining the sheets in a straight path during the feeding operation. The reliability of the sheet feeding system is an important characteristic of the system, and makes departures from the designs of conventional systems difficult to consider.
The advent of high speed photocopiers means that transfer sheets are used at a rapid rate, so that storage of a large number of sheets in the sheet delivery system is desirable to avoid the inconvenience of frequent loading. It is difficult to calibrate the springs and arrange for a constant feed position for the top sheet of a large stack of paper (e.g., one containing two reams of paper) in a system like the one referred to above. Generally a much more elaborate system is required than the biased plate system to maintain the top sheet of a large stack of stored sheets at a fixed feed position. The more elaborate such a system is, the more expensive and difficult to maintain it is. Many copiers have cassettes with holding capacities of only 250 to 500 sheets because that is the limit of what can be provided inexpensively. To the extent that an inexpensive larger capacity apparatus (one holding up to 1000 sheets, for example) may be devised, it would be desirable to have such an apparatus that can replace cassettes in existing photocopiers.
In patent application Ser. No. 76,575, filed Sept. 18, 1979, now abandoned in favor of continuation application Ser. No. 284,234, assigned to the assignee of the present application, a sheet delivery system responding to these problems is disclosed. In the system disclosed therein, the front end of a pivoting plate on which a sheet stack is supported rises under the influence of springs, but not enough to wholly compensate for the reduction in stack size. Accordingly, the feed rollers and sheet stripping mechanism are arranged to drop down to lower operating positions as the stack diminishes in height. As the pivoting plate of that system pivots about its rear edge, the stack size is reduced, the angle increases between the plate and the horizontal, and the angle at which the top sheet of the stack approaches the feeding mechanism also increases. The sheet stripping mechanism of the system has to be able to function, therefore, on top sheets approaching it through a range of angular positions.
Accordingly, principle objects of this invention are to provide a large capacity sheet delivery apparatus, for use with a device such as a photocopier, that has increased reliability in sheet delivery, and is also simple and inexpensive to manufacture and maintain.
Another object of the invention is to increase the sheet storage capacity of an existing photocopier by providing an add-on apparatus that is simple and inexpensive as well as reliable.
Other objects will be apparent in the folowing description and will be either explicitly stated or be inherent in the description.